(1) Field of the Invention
This invention relates in general to all roofing and in particular to a novel ventilating element in a roofing arrangement employing the same for ventilating a built-up roof.
(2) Description of the Prior Art
A conventional built-up, flat roof system is composed of at least one layer of a water impermeable roof membrane. The roof membrane is positioned over a roof deck which provides the bottom structure of the roof system over which the final outer roof treatment is placed. One such type of built-up roofing includes a number of layers of asphalt impregnated roofing felt layered on a sheathing of insulating board or sometimes directly onto the roof deck. The asphalt impregnated roofing felt provides a waterproof barrier, thereby preventing the entrance of water into the roof system. An alternative construction includes laying down a polyurethane foam layer of sufficient thickness to provide for some thermal insulation and, after the foam has had an opportunity to cure, laying down a water impermeable membrane over the top of the polyurethane foam layer.
A serious difficulty often associated with such built-up roofing is the presence of blisters forming under the waterproof membrane. Blisters are primarily caused by the expansion of air and water vapor which is trapped underneath the membrane, between adjacent layers of the asphalt impregnated roofing felt, or from the roof deck itself. The blisters eventually will rupture and allow additional water to enter the roofing. This moisture will first degrade the insulating value of the roofing and, over a period of time, cause structural failure of the roofing support system.
Because it is virtually impossible to prevent moisture from collecting in a built-up roof, attempts have been made to provide for its escape after it has become entrapped. One method of permitting the escape of air and water vapor involves using a special ventilating felt of a different composition than the usual felts. The special felt may include either a plurality of channels within it (See, for example, Long U.S. Pat. Nos. 3,387,420 to Long or 4,397,126 to Nelson) or, in the alternative, may include integrally therewith or molded therein a series of inner-connected water carrying channels disposed within the felt (See, for example, Glicksman U.S. Pat. No. 4,534,119 to Glickson). Finally, a double cuspated surface plastic film has been used to provide a multitude of venting air passages under the roof covering (see Friesen U.S. Pat. No. 4,538,388 to Friesen).
While these systems are effective in the case of roof systems using impregnated roofing felt, they are not particularly adaptable for roofing systems in which polyurethane foam is utilized or for repair of older roof systems in which only additional layers of mastic are added over the old roof. In addition, such systems add both considerable expense for materials as well as unnecessary weight to the roofing system.
It has then become desirable to develop a ventilating element for a built-up roof system that will permit harmful water vapor trapped in the roof system to escape while at the same time eliminate the prior art problems of unnecessary weight and specialized systems associated with the prior art.